1. Field of the Invention
This invention relates in general to compositions of matter and more particularly, to compositions useful as biomaterials. The compositions comprise organosilicon monomers (preferably silorane monomers) and chemical curing systems or dual chemical/light curing systems. The compositions may include one or more tetraoxaspiro[5.5]undecanes (“TOSUs”) and/or fillers. Photoacids, photosensitizers and/or a reaction promoters (electron donors) may also be included in the composition. The polymerizable compositions of the present invention are useful for a variety of applications, including use as biomaterials, for example as bone cements, bone stabilizers, dental composites, crowns, and the like.
2. Description of Related Art
Bone fractures are suffered by nearly six million Americans each year. The common methods for fracture stabilization such as casts, splinting, intramedullary pinning, and external fixation, do have their drawbacks especially with regards to stabilization of small and growing bones. In addition to hampering soft tissue management, splinting cannot adequately stabilize highly unstable fractures or injuries with considerable bone loss. There must be sufficient bone cortex to support pin stabilization, especially with epiphyseal (near the end of the bone) fractures. These fractures lacking sufficient cortex often require pin placement across the adjacent joint leading to joint stiffness. Fractures in children can be further complicated by rapid growth of bone, thus requiring continual adjustment of the stabilization technique.
Frequently, bone cement has been used to stabilize fractures. The current bone cements are methacrylate based systems packaged in two components. The powder contains a mixture of polymethyl methacrylate (“PMMA”), methyl methacrylate-styrene-copolymer, and a radio opacifier (either barium sulfate or zirconium oxide). The second component is a liquid monomer typically containing methyl methacrylate, N,N-dimethyl-p-toluidine, and hydroquinone. The cure time of commercially available PMMA bone cements ranges from 6 to 22 minutes, and reaches a peak exotherm from 75 to 110° C. Thermal finite element models have found temperatures in excess of 60° C. at the bone cement and cancellous bone interface. The fracture toughness of bone cements generally ranges from 1.0 to 1.5 MPa/m2. Flexural strength of bone cements ranges from 60 to 75 MPa and flexural modulus is between 2.2 and 3.3 GPa. Bone cements typically have a tensile strength of 50 to 60 MPa. These property values all meet or surpass the requirements described in ISO 5833 Implants for Surgery—Acrylic Resin Cements.
PMMA was first used in orthopedic surgery by Sir John Charnley for total hip arthroplasty in 1970 and the current formulation is essentially unchanged. Bone cement has different properties than other PMMA resins, due to both its additives and how it is prepared. Although the fully polymerized form of PMMA has good compatibility with human tissue, there are several drawbacks to its use. The monomer component is antigenic and induces severe toxicity, contraction with polymerization, and intense heat generation. These cements are highly exothermic, and have been shown to cause thermonecrosis in animal models. Bone cement typically has voids due to volatilization of the monomer during polymerization, which results in a porosity of 3 to 11%. Pressurization of the resin, as occurs during implantation of a femoral bone, can raise the boiling point of the monomer and decrease pore formation and size, but can be problematic if air is carried into the resin during implantation. The volume contracts by 5-7% during polymerization. This causes internal pores which can serve as crack initiators. The heat generated is determined by thickness/weight of resin, with a peak temperature rise in the range of 75-85° C. Although PMMA is biologically compatible, the monomer is an irritant and possible carcinogen. Unreacted residual monomer, approximately 3% after one hour, is present in the hardened polymer and can affect strength while leaching out into tissues, potentially causing hypotension, a common problem which can lead to possible cardiopulmonary events. The bone-resin interface is achieved by mechanical interlock, because the PMMA has no inherent adhesive properties. Dense bone causes less resin penetration, which can be an issue for resin fixation in younger patients such as military recruits whose bone structure may have less trabecular porosity. Use of bone cement in hip arthroplasties has also been shown to increase systematic levels of gamma-glutamytransperptidase (“GGTP”), which can result in anorexia and nausea/vomiting. Other drawbacks to the use of PMMA-based cements include lack of bioactivity, volumetric shrinkage upon polymerization, toxicity of the activator (N,N-dimethyl-p-toluidine) and possible involvement of the radiopacifier in third body wear. Therefore, it would be desirable to provide an alternative bone cement to PMMA-based cement.